[LoopUnroll] Fix potentially incorrect SCEV invalidation in UnrollRuntime

Current runtime unrolling invalidates parent loop saying that it might have changed
after the inner loop has changed, but it doesn't bother to do the same to its parents.
With patch rL329047, SCEV becomes much smarter about calculation of exit counts for
outer loops. We might need to invalidate not only the immediate parent, but also
any of its parents as well.

There is no clear evidence that there is some miscompile happening because of this
(at least I don't have such test), but the common sense says that the current code
is wrong.

Differential Revision: https://reviews.llvm.org/D45940
Reviewed By: chandlerc

llvm-svn: 330577